This application claims priority from Korean Patent Application No. 2002-71391, filed on Nov. 16, 2002, in the Korean Intellectual Property Office, the contents of which are incorporated herein in their entirety by reference.
1. Field of the Invention
The present invention relates to a super twisted nematic (STN) liquid crystal display (LCD) driver, and more particularly, to an STN LCD driver using a frame rate control (FRC) technique as a driving method.
2. Description of the Related Art
In a super twisted nematic (STN) liquid crystal display (LCD) driver using an iAPT or APT method, pulse width modulation, frame rate control (FRC), or a combination thereof is widely used to present gray scales and colors.
FIG. 1 is a table showing levels of a super twisted nematic (STN) liquid crystal driving voltage in response to a level of a liquid crystal polarity inversion signal M in a FRC method.
The liquid crystal polarity inversion signal M is a periodic signal used to prevent liquid crystal from solidifying. That is, the level of the liquid crystal polarity inversion signal M has to be periodically inverted so as to prevent the liquid crystal from solidifying. After the level of the liquid crystal polarity inversion signal M is inverted, levels of a segment voltage VSEG driving a column electrode of an STN liquid crystal and a corn voltage VCOM driving a row electrode of the STN liquid crystal are also inverted.
Referring to FIG. 1, if the level of the liquid crystal polarity inversion signal M is high, the selection voltage level of the segment voltage VSEG is V0. If the level of the liquid crystal polarity inversion signal M is low, the non-selection voltage level of the segment voltage VSEG is Vss.
A relationship between the voltage levels of the segment voltage VSEG can be expressed by V0>V2>V3>Vss.
If the level of the liquid crystal polarity inversion signal M is high, a non-selection voltage level of the segment voltage VSEG is V2. If the level of the liquid crystal polarity inversion signal M is low, the non-selection voltage level of the segment voltage VSEG is V3.
Similarly to the voltage level of the segment voltage VSEG, the voltage level of the corn voltage VCOM is also inverted in response to the level of the liquid crystal polarity inversion signal M.
If the level of the segment voltage VSEG or the corn voltage VCOM are the selection voltage level, the liquid crystal is turned on and presented as black. If the level of the segment voltage VSEG or the corn voltage VCOM are the non-selection voltage level, the liquid crystal is turned off and presented as white.
Yet, if the non-selection voltage levels V2 and V3 of the segment voltage VSEG are not used the same number of times in the FRC method, cross talk occurs in the liquid crystal.
FIGS. 2A through 2D are waveforms of a segment voltage VSEG in a conventional STN LCD driving method using a 3FRC method.
A frame in an nFRC method is comprised of n of sub frames. Thus, a frame in the 3FRC method is comprised of three sub frames. Pixels at the right side of FIGS. 2A through 2D present different gray scales of the liquid crystal.
That is, FIG. 2A shows the liquid crystal presenting black, FIG. 2B shows the liquid crystal presenting dark gray, FIG. 2C shows the liquid crystal presenting light gray, and FIG. 2D shows the liquid crystal presenting white.
At each sub frame, the level of the liquid crystal inversion signal M is inverted. Thus, the same signal is repeated once every six sub frames.
In the first sub frame, the level of the liquid crystal polarity inversion signal M is high. In the second sub frame, the level of the liquid crystal polarity inversion signal M is inverted, i.e., low. Accordingly, in order to present the liquid crystal as dark gray, two sub frames have to present black, and one sub frame has to present white as shown in FIG. 2B.
Referring to FIG. 2B, in order to present the liquid crystal as dark gray, the level of the segment voltage VSEG is V0 which is the selection voltage level in the first sub frame where the level of the liquid crystal inversion signal M is high. The level of the segment voltage VSEG is Vss which is the non-selection level in the second sub frame where the level of the liquid crystal inversion signal M is low. The level of the segment voltage. VSEG is V2 which is the non-selection level in the third sub frame where the level of the liquid crystal inversion signal M is high. When such sub frames are consecutively repeated by a period over 60Hz, human eyes recognize these as dark gray. Here, since the selection voltage levels V0 and Vss and the non-selection voltage levels V2 and V3 are used the same number of times in total 6 sub frames, cross talk does not occur in the liquid crystal.
FIGS. 3A through 3E are waveforms of a segment voltage VSEG in a conventional STN LCD driving method using a 4FRC method.
A frame is comprised of four sub frames. Referring to FIG. 3B, only V3 is used as the non-selection voltage level, and V2 is not used as the non-selection voltage level. In addition, the selection voltage level V0 is more frequently used than the other non-selection voltage level Vss.
Referring to FIG. 3C, the selection voltage levels V0 and Vss are used the same number of times, and the non-selection voltage levels V2 and V3 are used the same number of times also.
Referring to FIG. 3D, the non-selection voltage level V3 is more frequently used than the non-selection other voltage level V2. Only V0 is used as the selection voltage level, and Vss is not used as the selection voltage level.
Thus, cross talk occurs in the liquid crystal in the waveforms of FIGS. 3B and 3D, and does not occur in the liquid crystal in the waveform of FIG. 3C.
FIGS. 4A through 4E are waveforms of a segment voltage in an STN LCD driving method using an N-line inversion method.
The N-line inversion method is used to prevent cross talk shown in FIGS. 3A through 4E from occurring. The N-line inversion method includes dividing one frame into N sub frames and inverting the level of the liquid crystal inversion signal M in each sub frame.
Referring to FIGS. 4A through 4E, the selection voltage level or the non-selection voltage level are used the same number of times. Therefore, cross talk can be prevented from occurring in the liquid crystal.
However, the N-line inversion method increases the number of transitions of the level of the segment voltage VSEG. Thus, power consumption also increases. That is, the nFRC method has a disadvantage in that cross talk occurs, and the N-line inversion method has a disadvantage of increased power consumption.